This invention relates to polyurethane foams containing an amylaceous material, such as a cereal starch or flour, and a process for making same.
Polyurethane foams typically are prepared by reacting one or more active hydrogen-containing organic compounds, particularly polyols such as polyethers and polyhydroxyl-containing polyesters, with a polyfunctional isocyanate in the presence of a blowing agent and, optionally, in the presence of catalysts, stabilizers, surfactants, crosslinking agents, plasticizers, dyes, fillers and pigments. There are four general types of polyurethane foams: (1) rigid, (2) semi-rigid or semi-flexible, (3) flexible, and (4) high resilience. The type of foam produced is determined primarily by the molecular weight and functionality of the active hydrogen-containing organic compound.
Considerable effort has been devoted to reduce the cost of preparing these foams. Towards this end, efforts have been made to employ low cost starches as a reactant to replace a portion of the more expensive polyol or other active hydrogen-containing organic compounds typically employed in the above types of foams. The direction addition of starch along with the other foam-forming ingredients, such as in a "one-shot" process, generally has not been satisfactory because the resultant foams have poor physical properties.
Examples of prior art processes employing starch in polyurethane foams include U.S. Pat. No. 2,908,657 (Boggs) which discloses the incorporation of various starches into a polyurethane foam by adding the starch to a completely reacted liquid reaction product of a polyester and a diisocyanate. Boggs teaches it is essential that the polyester and diisocyanate be mixed and completely reacted before the starch is incorporated; otherwise, the resultant foam product has poor tear resistance and tensile strength. U.S. Pat. No. 3,004,934 (Dosmann et al) discloses the addition of starches to a liquid prepolymer formed by reacting an organic polyisocyanate with a polyether or a polyester to produce foams having predominantly closed cells. German Patent application No. 2,448,216, published Apr. 10, 1975, discloses the use of a carbohydrate filler, such as starch, in polyurethane foams employing a polyisocyanate having a relatively high functionality.
Prior attempts to overcome the difficulties associated with incorporating starches directly into polyurethane foams include using oxyalkylated starches, using polyhydroxypolyoxyalkylene ethers formed by reacting starch with a polyhydric alcohol in the presence of an acid catalyst and then oxyalkylating the resultant reaction mixture as disclosed in U.S. Pat. Nos. 3,227,213 (Fuzesi) and 3,402,170 (Fuzesi et al), and using alkoxylated starch hydrolysates as disclosed in U.S. Pat. No. 3,600,338 (Molotsky).
U.S. Pat. Nos. 3,165,508 (Otey et al), 3,405,080 (Otey et al), and 3,655,590 (Moss et al) disclose the use of starch-based polyols in the production of rigid and flexible polyurethane foams. Attention is also directed to U.S. Pat. Nos. 3,674,717 (Fuzesi et al), and 3,957,702 (Molotsky et al) which disclose the use of phosphorous derivatives of starch polyethers or starch polyether hydrolysates to produce flame retardant polyurethane foams. An article entitled "Rigid Urethane Foam Extended With Starch", in the Journal of Cellular Plastics, August 1967, discloses that rigid polyurethane foams containing starch and made with conventional polyethers do not have acceptable physical properties.
Any moisture present in the starch can participate in the polyisocyanate reaction, thereby upsetting the desired balanced stoichiometry between the reactive hydrogen equivalents and the isocyanate equivalents in the foam-forming reaction system with a resultant change in the properties of the final product. Therefore, it is highly desirable to utilize starches which are substantially free of moisture or have been dried to constant moisture level. It has been found that polyurethane foams containing an amylaceous material which has been dried in an oxidizing atmosphere cannot be cured under normal conditions and usually have unacceptably low strength properties. The same phenomenon has been observed for foams containing an amylaceous material, particularly cereal starches and flours, which have been stored for extended time periods under atmospheric conditions.